The most commonly used hydraulic cement is portland cement, made by burning (calcining) crushed limestone, clay, alumina and silicates until the mass is nearly fused. This material, called clinker, is then combined with gypsum (actually anhydrite--calcium sulfate--CaSO.sub.4) and ground into a fine powder. Mixed with water, the pulverulent materials undergo a rapid chemical reaction called hydration (thus the term "hydraulic cement"), forming hydroxide compounds that hydrolyze the silicate components of the mixture into amorphous phases that eventually fuse into a solid mass. Hydraulic cement has been known since ancient times as the primary bonding material that holds together the aggregates in concrete. It is also, for all practical purposes, a form of chemically bonded ceramic.
The preparation of portland cement requires mining, refining and transportation of the various raw materials. These activities consume large amounts of energy and produce substantial quantities of carbon dioxide, as do the calcining and grinding processes. The carbon dioxide so produced is disgorged into the atmosphere, contributing to global warming.
So-called fly ash is co-produced during the burning of coal, wood and many other types of organic or fossilized hydrocarbon fuels as vaporized (gas-phase), incombustible, inorganic contaminants condense to form particles, and these particles then further coagulate to form fine spherical and cenospherical aggregation particulates during the rapid cooling of the flue gas and mineral matter. The condensation occurs in the presence of water vapor from various sources, including combustion, as well as carbon dioxide, nitrogen, nitrogen oxides and sulfur oxides (called NO.sub.x and SO.sub.x, respectively). In most commercial combustion programs, the largest fraction of these particulates generally is captured from the flue gas stream in pollution control equipment and transported to specially constructed landfills or returned to the mines from which the coal originated, where they are deposited as waste. If its unburned carbon content is less than 1%, fly ash, being an artificial pozzolan, may be used as a cement additive to reduce the high relative pH of the pore water of concrete made of portland cement and aggregate. This practice is often desirable to prevent or mitigate a reaction (called the Alkali-Silica Reaction or "ASR") between the cement and siliceous aggregates. ASR can cause early concrete deterioration. Fly ash may sometimes also be used as a filler for cast and molded plastics made of catalyzed resins or thermoplastics.
The most desirable types of fly ash for use as a concrete mixture exhibit relatively high reactivity in portland cement. That is, they will bind significant amounts of hydroxide, but not inhibit the rate of cement hydration and, in certain cases, may even accelerate it. This occurs in one of two ways. First, the ash may contain relatively little calcium and/or magnesium oxide, but will express most of its silica content in a glass phase such as an amorphous gehlenite (rather than as crystalline siliceous minerals such as mullite). Second, the ash may contain large proportions of calcium and/or magnesium oxide which, when combined with water, will act as a cementing agent in its own right. To maintain high reactivity it is desirable to have little or no unburned carbon (called Loss On Ignition or "LOI") present in the ash. Crystalline carbon also causes early strength deterioration in cements. Because of its dark color, fly ash with higher LOI content is also undesirable for use as a filler for cast or molded plastics.
Ideally, it would be desirable to produce hydraulic cement or low-carbon/high-amorphous-phase ash without expending additional energy for calcining limestone, for obtaining and transporting raw materials and without releasing the additional carbon dioxide resulting from these activities into the atmosphere. Instead, there would be a process that would transform into useful products the large dusty waste streams produced by commercial solid fuel combustion (e.g., in electric power plants) and that would eliminate reliance upon extraneous materials and energy. The process would be even more desirable if, in its operation, it also increased combustion efficiency.
Like cement kilns, organic- and fossilized-hydrocarbon-fired electric power plants produce sintered inorganic materials: smoke, bottom ash, fly ash, fouling and slag. Such products are considered byproducts and are usually treated as wastes. Often referred to as "dirt-burners" in the electric industry, the combustion units in the boilers of coal-burning plants cause the mineral-matter impurities in the coal feed stream to vaporize into gases or near gases, after which they condense, coagulate and are quenched, transforming them into different minerals in the resulting "ash", much like the limestone/clay/bauxite feedstocks in cement plants are transformed into clinker by burning in cement kilns. In fact, the ashes produced by many electric power plants have chemistry, but not mineralogy, very close to that of portland cement. The result of coal burning is siliceous ash particulates that are later removed from the flue streams by electrostatic precipitators, fabric baghouses or capillary ceramic candle filters. The same can be said of combustors burning wood, rice hulls and other organic fuels.
With the intent that it will pass through the furnace, become calcined into lime, and subsequently act as a sorbent for sulfur oxides in the flue gas to form anhydrite (calcium sulfate--CaSO.sub.4), attempts have been made to introduce limestone or lime into coal pulverizers. Although this is effective to a limited extent, particularly in circulating fluidized bed powerplants, the limestone tends to melt, aggregate and clump in the lowest part of the bed at the bottom of the furnace or combustor, where it does not react and is discharged with the bottom ash. As large masses of limestone form, they attract the remaining free lime and form what are, for all practical purposes, metamorphic materials similar to marble, effectively stopping any dry scrubbing before it can start. If the lime were hydrolyzed as soon as it was calcined from limestone, the reaction with the sulfur oxides and the formation of anhydrite particulates in the flue stream would be accelerated, facilitating the dry scnibbing process. In addition to or in place of limestone, high-calcium fly ash has also been added to the coal feed stream with similar results, again, provided hydrolysis occurs.